The present invention relates to a pressure sensitive adhesive composition, and more particularly to the composition containing a polymer having a reactive silyl group, which has excellent releasability from a release paper.
A pressure sensitive adhesive is a kind of adhesive which is permanently tacky and can firmly adhere to a surface without the need of more than finger pressure. Examples of the use of pressure sensitive adhesive are in cellophane, masking and electrical tapes and in self-adhering label and sheet.
A pressure sensitive adhesive must have important properties that not only it can adhere to a surface of materials, but also it can easily peeled off from a desired surface of materials. For instance, in case that the pressure sensitive adhesive is applied to a self-adhering label or sheet, the pressure sensitive adhesive face is protected with a release paper. This release paper must be easily peeled off from the pressure sensitive adhesive face, that is, the release paper must not be teared or the pressure sensitive adhesive must not remain on the release paper. In case of a electrical tape, the back side of the pressure sensitive adhesive face must have the same properties that release papers have.
Examples of the release paper are, for instance, a paper treated with a silicone release agent or a fluorine compound, and the like. The releasability of the electrical tape is also improved by applying a silicone release agent or the like on the back side of the pressure sensitive agent.
Many pressure sensitive adhesives are excellent in the releasability from the above-mentioned release paper or back side of tape. As the pressure sensitive adhesive composition having excellent releasability, there is known a composition such as rubber pressure sensitive adhesive composition prepared by admixing a natural rubber or a synthetic rubber with a tackifier, or a polyacrylate pressure sensitive adhesive composition containing a polyacrylate prepared by copolymerizing acrylic acid esters. However, these pressure sensitive adhesives have the disadvantage that the heat resistance is poor. That is, the foundamental properties for pressure sensitive adhesives such as adhesive strength and holding power are remarkably lowered with increase in temperature. Because natural or synthetic rubber pressure sensitive adhesives are adhesives of non-crosslinking type, it is expected that they are poor in heat resistance. Even with respect to the polyacrylate pressure sensitive adhesive, in spite that the polyacrylate is cross-linked by a cross-linking agent such as an isocyanate, the heat resistance of the cross-linked polyacrylate itself is not always sufficient.
On the other hand, as a pressure sensitive adhesive having excellent heat resistance, a silicone pressure sensitive adhesive prepared by admixing a dimethyl polysiloxane rubber as a base material with a silicone resin has been known. However, the silicone pressure sensitive adhesive has the disadvantages that it has a high adhesive strength even to a nonpolar substance such as polytetrafluoroethylene and also it has an insufficient releasability to a silicone release paper, because both the silicone pressure sensitive adhesive and the silicone release paper include polysiloxane and the silicone release paper include polysiloxane and accordingly are compatible with each other.
Recently, it is proposed to prepare a pressure sensitive adhesive composition having excellent heat resistance by introducing a reactive silicon-containing group (hereinafter referred to as "reactive silicon group") into a organic polymer, thus heat stable siloxane linkages are formed by condensation reaction of the reactive silicon group. Such a pressure sensitive adhesive is disclosed, for instance, in U.S. Pat. No. 4,463,115 to the present inventors. However, this pressure sensitive adhesive of cross-linking type has the disadvantage that its releasability from a silicone release paper is poor like the above-mentioned silicone pressure sensitive adhesive in spite that this pressure sensitive adhesive has essentially a main chain of organic skeleton. That is, when this pressure sensitive adhesive is laminated to a release paper or film or when this pressure sensitive adhesive is coated onto a substrate the backside of which is coated with a silicone release agent and the coated substrate is wound, the peeling resistance between the pressure sensitive adhesive layer and the release paper or film rises with the lapse of time and the release paper cannot be peeled off due to breaking of the paper in the worst case.
An object of the invention is to provide a pressure sensitive adhesive composition having excellent heat resistance and releasability from substrates, e.g. a release paper coated with a silicone release agent.
This and other objects of the present invention will become apparent from the description hereinafter.